In recent years, the popularity of wireless communication devices, such as cellular telephones for example, has increased exponentially, to the point where such devices are practically household items.
Providers of wireless telecommunication services typically divide a geographic region into a plurality of adjacent hexagonally-shaped “cells”, each cell having a central base station for transmitting and receiving to and from any wireless communication devices which are presently located within the cell. Many older base stations include omnidirectional transceivers for transmitting and receiving in all directions within the cell. However, omnidirectional cell antennae typically have relatively low call capacities, and are overly susceptible to interference, as interference on a given frequency channel will prevent use of that channel anywhere within the cell, even if the interference is actually affecting only a relatively narrow angular region within the cell.
To address these difficulties, wireless telecommunication service providers have resorted to “sectoring”, in which each cell is further divided into a number of “sectors”, such as three 120° sectors for example. Three directional antennae at the base station are used to transmit and receive signals only within the confines of a respective corresponding sector. In a sectored cell, interference on a particular frequency channel in one sector will not necessarily preclude use of that channel in another sector of the cell. The total number of frequency channels available for use within the cell is typically divided or allocated among the sectors, either on a fixed basis or preferably, dynamically.
More recently, as an extension of the above principles, service providers have implemented multi-beam cellular communications systems, in which each sector is further subdivided into a number of “beams”, such as four 30° beams in each of the three sectors of the cell, for example. In this example, the base station would include twelve directional antennae, each antenna being configured to transmit and receive signals only within the physical confines of a particular corresponding one of the twelve beams into which the cell is divided.
It will be appreciated that the number of frequency channels available on any given cell is limited, due to the allocated spectrum for cellular communications and due to interference requirements which prevent a given cell from using the same set of frequencies as an adjacent cell. Thus, the base station has a limited number of frequencies which it must allocate or assign to the various beams within the cell. However, existing channel allocation methods do not appear to be suitable for multi-beam systems.
Fixed Channel Allocation (FCA) involves allocating a fixed number of frequency channels to each beam, such as eight channels per beam, for example. However, FCA fails to account for the dynamic nature of signal traffic or traffic demand within the cell. For example, at a given instant, only half of the available channels at the cell might be in use, but a particular user might not be able to place a call, simply because the user happened to be physically located within a beam in which all eight pre-allocated channels were in use, even though an adjacent beam might have none of its channels in use. Further, if the number of beams is increased, the number of fixed channels allocated per beam decreases, thus decreasing the trunking efficiency.
A number of dynamic channel allocation schemes exist. Some such schemes involve scanning available frequency channels for interference to determine channel quality, and assigning, to a sector for example, the channel having the highest channel quality within the sector. However, in multi-beam systems, because the beams are narrow and in close physical proximity to one another, channel quality is highly correlated among adjacent beams. In other words, the same frequency channels which are clear in one beam often tend to be equally clear in an adjacent beam, and the channels which are noisy in one beam are often equally noisy in the adjacent beam. Thus, in a multi-beam system, channel quality alone is not sufficient to resolve competing demands from adjacent beams for a new frequency channel.
In addition, as a user of a wireless telecommunications device such as a cellular telephone moves from one region to another adjacent region, such as moving from one beam to an adjacent beam within a cell for example, it is necessary for the wireless telecommunication service provider to perform an “inter-beam hand-off” or transfer the call to a new frequency channel in the adjacent region, and to signal the cellular telephone to re-tune to the new channel. Although such a hand-off ideally occurs sufficiently quickly that it is not noticeable to the user, occasionally a brief “mute” or signal interruption will occur, or the call may even be blocked or lost. This difficulty is aggravated in a multi-beam system, in which the regions (beams) are smaller, with the result that a larger number of hand-offs are required to track the user of the wireless communications device over a given range of movement. The increase in hand-offs may also result in increased signaling.
Thus, there is a need for a better way of assigning frequency channels to a beam in a multi-beam cell. In addition, there is a need for a better way of communicating with a source moving from a first region to a second region.